This invention relates to electric machines with superconducting rotor windings in general and more particularly to a coolant conducting head for such a machine.
A coolant connecting head for an electric machine which comprises a rotor supported rotatably about an axis with a superconducting winding which can be deep cooled by a coolant, with a device for feeding the coolant from a non-rotating coolant line part into a rotating coolant line part connected to the rotor, which contains at least one sealing device which serves for sealing off a space formed between said line parts and which allows, in the operating condition of the machine, a predetermined leakage rate which depends on the pressure difference prevailing therein is described in the literature reference "Advances in Cryogenic Engineering," vol. 23, New York, 1978, pages 125 to 131.
For cooling a superconducting winding in the rotor of an electric machine, especially of a turbo-generator, devices must be provided for conducting a coolant between the rotor and stationary connecting lines. The coolant, for instance, liquid or gaseous helium, is fed to the winding in the rotor or is discharged again therefrom through such a device. The machine is therefore provided with a connecting head which contains a suitable transfer device. The design of this device, also called a coupler, is difficult, especially with respect to low thermal losses of the coolant loop for the superconducting winding and with respect to low leakage rates. For, the coupler must have rotary seals with relatively little friction, which seal the coolant, particularly the liquid helium, on the one hand, from the outside and which, on the other hand, separate the inlet side from the outlet side within the rotating system. The sealing devices required therefor must, in addition, permit radial and axial shaft clearance and operate without trouble for extended periods of time, e.g., several years.
The known coolant connecting head contains a suitable helium coupler. It is provided with a hollow cylindrical stationary housing, into the interior of which a tubular feed line extends. The open end of this feed line is concentrically surrounded by the end section of a conduit firmly connected to the rotating parts of the machine, so that an annular space is formed between these two tubes. The concentric position of the two tube sections is ensured via bearings provided for this purpose. To prevent the helium which is fed in from escaping through the space to the outside, a seal with a predetermined gap is provided, for instance (see FIG. 3). The dimensions of the gap are made so that contact between rotating and stationary parts of the transfer device is prevented in the event of radial and axial vibrations of the rotor shaft so as to preclude friction heat and wear of parts of the seal. In general, the seal gap cannot be made arbitrarily small and a certain leakage rate must be tolerated.
The leakage rates in sealing devices with gaps between rotating and non-rotating, largely contactless sealing parts can be kept relatively small if a so-called self-pumping effect in thermal-syphon loops is utilized for the cooling system of the winding of a rotor (cf., the journal "Cryogenics," July, 1977, pages 429 to 433, and DE-OS No. 25 30 100). For then, the coolant only needs to be fed into the rotor at the transfer device with a relatively small overpressure, so that a correspondingly small pressure difference prevails at the sealing devices. This is true, however, only for the cooling system in the cooled down condition. For, during the cooling down phase of the winding and especially during the starting phase, the flow resistance of the cooling system is still very high. Since, to achieve acceptable cooling down times, considerably higher coolant throughputs are required than in the cooled down state, i.e., the coolant must be fed into the cooling system with correspondingly higher pressure during these times, correspondingly larger leakage losses result in these sealing devices during this cooling down phase.